Amphiphilic core–shell nanospheres obtained by intramicellar shell crosslinking of polymer micelles with poly(ethylene oxide) linkers

Abstract

Intramicellar crosslinking of the polymer chains within the shells of polystyrene-b-poly(acrylic acid) micelles by reaction with difunctional poly(ethylene oxide) afforded unimolecular amphiphilic core–shell nanospheres (50 nm hydrodynamic radius); the resulting surface hydrogel layer gives an approximate fivefold increase of particle volume from the dry state to aqueous solution.

References

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11. The PS-b-PAA was prepared by living atom transfer radical polymerization of styrene and then methyl acrylate, followed by HCl catalyzed hydrolysis of the methyl ester functionalities. The polystyrene block was prepared by polymerization of styrene, using amixture of copper(I) bromide (0.8 mol%), 4,4′-di(5-nonyl)-2,2-bipyridine (1.5 mol%) and 1-bromoethylbenzene (0.8 mol%). This bromo-terminated polystyrene (0.8 mol%) was isolated by precipitation into methanol. The dried bromo-terminated polystyrene was then used to initiate the polymerization of methyl acrylate, in the presence of copper(I) bromide (0.8 mol%) and 4,4′-di(5-nonyl)-2,2′-bipyridine (1.7 mol%). The diblock copolymer was made amphiphilic by hydrolysis of the methyl ester functionalities along the poly(methyl acrylate) block in HCl–dioxane. The PS block was of M_n= 12 500, determined from GPC calibrated with polystyrene standards. Based on the ratio of integration values for resonances of respective protons in the ¹H NMR spectrum, the average numbers of styrene units and acrylic acid units in the PS-b-PAA copolymer were determined to both be approximately 120. J. S. Wang, D. Greszta and K. Matyjaszewski, ACS Polym. Mater. Sci. Eng., 1995, 73, 416.
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13. The single alkyl bromide located at the chain end of each polymer chain resulting from the ATRP chemistry may also undergo reaction with the diamino crosslinkers.
14. The sizes of SCK particles of 1 shown by TEM cannot be taken as the compact dry-state diameters, owing to distortion of the PEO-containing corona of the nanospheres upon drying on the carbon-coated copper grid substrate. Therefore, the SCK size was predicted by adding the volume of the precursor micelle to the additional volume from the linkers (calculated from the product of moles of linker, linker molecular mass and linker density), followed by conversion to the diameter for a spherical SCK. The 29 nm calculated diameter of 2 agrees with the particle sizes observed in the TEM image.
15. T_m of PEO–(NH₂)₂(M_w= 3400) is 45 °C.